Hmm…
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AC/DC, Whole Lotta Rosie
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Jim Croce, Roller Derby Queen
There are plenty more, but that’s a reasonable start!
Thunder Road - Bruce Springsteen:
Show a little faith, there’s magic in the night
You ain’t a beauty, but hey you’re alright
Oh, and that’s alright with me
The Kinks: “When I Turn Off the Living Room Light”
(I can’t forgive that opening line, though. Why, Ray?)
Joe Cocker’s “You Are So Beautiful.”
Nice sentiment, til you get to the “to me” part. 
"If You Want to Be Happy" by Jimmy Soul
If you want be happy for the rest of your life
Never make a pretty woman your wife
So from my personal point of view
Get an ugly girl to marry you.
The Most Beautiful Girl (In The Room) by Flight of the Conchords.
I always hoped someone would write a love song for Allison Gross. Song always made me feel sorry for her.
“If You Could See Her” from Cabaret.
She’s My Girl by Tom Lehrer. 
Queen - Fat Bottomed Girls
Superballs by the Insane Clown Posse.
“You know that bag lady skank that always bothers you?
I’m here to fuck her yo… it’s what I do!”
The Chelsea Hotel No. 2
Supposedly about Leonard Cohen and Janis Joplin, neither of which were particularly physically attractive.
Dave Clark Five’s “You Got What It Takes”:
You don’t live in a beautiful place
And you don’t dress in the best of taste
Oh! Nature didn’t give you such a beautiful face
But baby, you got what it takes
You Got What It Takes - The Dave Clark Five
‘you don’t live in a beautiful place, you don’t dress in the best of taste, nature didn’t give you such a beautiful face, but baby, you got what it takes - oh! - to satisfy…
you got what it takes for me!’
always loved this song. Take that, super-models and movie stars, us homely folks find love, too!
I always got the feeling that Maggie May was a few years past her prime…
My Funny Valentine
Nice choice. Probably the most subversive love song ever.
Springtime for Hitler (we needed an “unattractive guy” song, after all…)
Thunder Road